Abstract: A seat back frame of automotive seat, which has an armrest connection point in one of the two side frame members thereof, to which an armrest may be mounted. A reinforcing element is removably secured between such one of the two side frame members and a lower frame member of the seat back frame. The reinforcing element is so formed to have a long portion and a short vertical portion extending vertically and continuously from the long portion. Such long portion and short vertical portion are removably secured to the foregoing lower frame member and the foregoing one of the two side frame members, respectively.